Here are some suggestions:

HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

If I instead pull up the demo lesson (listed under recent lessons), I can click "Next Lesson", which asks me my experience level, but clicking continue just brings me back to the first lesson again. (And clicking next lesson asks me again about my experience level).

Not from a bookmark. I go to www.chess.com, hover over the learn button, select chess mentor, and then click "start training".

And as mentioned, if I instead pull up a previous lesson, it works fine for that lesson, but clicking next lesson does not work--it kept looping between my only previous lesson and the page asking me about my chess experience level.

On my macbook clicking "start training" worked fine. I haven't tried on my desktop since--I imagine bringing up a previous lesson may loop in some new and interesting way now that there are multiple previous lessons, instead of just the single demo lesson I was able to bring up.

Also of note--during the demo lesson on my windows desktop, after successful mate, I got an error popup with an ajax error--I posted in the other thread, the one about runtime errors. I do not get any errors on my macbook.

Hello, i have a problem with chess mentor, and i dont know how to contact the chess stuff for resolve it. I cant do the 3 free lessons per day, because when i try do the daily lessons, the chess mentos shows me the same lessons I've already done, always the same one!!!